Fair Value Measurements and Investments - Schedule of Amortized Cost, Unrealized Gains and Losses, and Fair Value of Available-for-sale Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,412	$ 5,806
Gross Unrealized Gains	44	1
Gross Unrealized Losses	(5)	(82)
Fair Value	5,451	5,725
U.S. government-sponsored debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	492	1,109
Gross Unrealized Gains	4	1
Gross Unrealized Losses	0	(2)
Fair Value	496	1,108
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,920	4,697
Gross Unrealized Gains	40	0
Gross Unrealized Losses	(5)	(80)
Fair Value	$ 4,955	$ 4,617